1. A nautical signal consists of six flags arranged vertically on a flagpole. If a sailor has three red flags, two blue flags, and one white flag, how many different signals are possible?
2. A psychology experiment observes groups of four individuals. In how many ways can an experimenter choose 5 groups of 4 from among 20 subjects?
3. During orientation, new students are divided into groups of five people. In how many ways can 4 groups be chosen from among 20 people?
4. In a certain month (of 30 days) it rains 10 days, snows 2 days, and is clear 18 days. In how many ways can such weather be distributed over the month?